The present invention is directed to an improved shape memory metal (SMM) actuator which exhibits reduced tendency for the actuator element to break at the point of attachment to and end fitting.
In a SMM actuator a SMM wire is typically attached to an end fitting with compressive clamping forces. Such compressive clamping force radially compress the end fitting, plastically deforming a surrounding clamp onto the SMM wire. This approach produces a large, radially directed, compressive, residual contact pressure that bonds the SMM wire to the end fitting through friction. This approach brings with it two detrimental effects that limit the life of the SMM wire at the attachment point.
The first of these effects is an increased stress from two separate causes. The end fitting by necessity is larger than the SMM wire, and there is therefore a change in cross-section at the transistion from the wire to the end fitting. This discontinuity in cross-sectional size causes an increase in stress at the connection above the average stress level. In addition, the end fitting and the clamp must grip the SMM wire hard enough to prevent slippage, and this gripping induces local compressive stresses in the SMM wire above the average stress. The present invention is not addressed to this stress problem.
The second detrimental effect is due to the much larger thermal mass and increased thermal conductivity of the end fitting. Typically, this is due simply to the larger size of the end fitting. The result is that the SMM wire near the end fitting remains at a much lower temperature than the central portion of the SMM wire when the SMM wire is heated during routine temperature cycling. The relatively large end fitting acts as a heat sink to draw heat out of the SMM wire. This effect is only felt by the wire for a few diameters of length away from the end fitting. Typically, wire that is more than about five diameters away from the end fitting is unaffected by the heat sink properties of the end fitting.
As is well known, SMM wire has two states, separated only by temperature. When cool, the SMM wire is in the martensitic state, in which the wire is relatively soft and easily deformed. When warmed above a transition temperature the SMM wire is transformed into the austenitic state, in which the wire is much stronger and stiffer than when in the martensitic state. When in the martensitic state, the SMM wire is deformed under relatively low load. When heated through its transition temperature, the SMM wire remembers its original shape and tends to return to that shape. In the process, it builds up stresses that oppose the original deformation, and the SMM wire can do work while returning to its original shape. SMM actuators often use SMM wire in tension (straight sections of wire) of torsion (helical coils of wire). The SMM wire is deformed while cool. When actuation is required, the wire is heated to a temperature above the transition temperature, usually by passing an electrical current through it. Such an electrical current imparts energy into the wire, in the form of heat, equally along the length of the wire. However, the end fittings conduct heat out of the end portions of the SMM wire, so that these end portions of SMM wire always remain below the transition temperature, in the martensitic state. During actuation, the stresses in the SMM wire rise as the wire does work. Since the SMM wire near the end fittings is relatively weak in comparison to the center portion which is in the austenitic state, the relatively soft portions of the SMM wire near the end fittings tend to elongate more than the central section of the wire, thereby reducing the cross-sectional area of the SMM wire in these martensitic regions. Such reductions in cross-sectional area often lead to early failure of the SMM wire adjacent to the end fittings.